Introduction
At ambient temperature and pressure, mercury (the elemental mercury) is a metal in form of heavy silvery-white liquid. It exists in three oxidation states: mercuric Hg+2, metallic Hg, and mercuros Hg2 +2. Organic and Inorganic compounds are formed by mercuric and mercuros states. The mercuros state is unstable under environmental conditions. Mercury exists in water, sediment, soil and biota only in inorganic mercuric salts and organomercurics.
In the Cold Vapour analysis, mercury from the digest sample is reduced with either SnCl2 or NaBH4 and then measured with a conventional cold vapour flameless atomic absorption spectrometry. The whole system is based on the principle of injection flow. Low sensitivity can be attributed to hardware or chemical factors. Deciding where the problem exists requires a basic understanding of the hydride technique.
The reduction equation is given below
E m+ +H (excess) →EHn+H2 (excess)
Where E is the analyte of interest and m may or may not be equal n.
Determination of Total Hg in Fish
(This is a lab report that is only compiled after the performance of the experiment).
Hg wavelength = 253nm
Lamp current = 4.0 mA
Slit width = 0.5 nm
Mass of the freeze-dried shark sample = 0.2g (an equivalent of 0.1g of swordfish)
Working solution = 100µgL-1 (ppb) Hg
Volume of each prepared standard = 50 mL
To calculate the concentration of Hg in the sample digest, let us consider the results below
Concentration of Hg = √ {(0.4 + 0.6 + 6.5 + 5.6 + 8 + 12) / (600)} = 0.235 mg/L
To calculate the Hg content in the original dried fish tissue in mg/kg, the formula below is used
Concentration of mercury, mg/kg = ((C1 - C2) x V x D.F) / D.W (Dry weight basis)
Where:
C1 = Concentration of metal in the sample (obtained from the calibration graph or direct read out from the instrument), mg/L (The typical detection limit for this method is 0.0002 mg/L)
C2 = Concentration of metal in the blank, mg/L
V = Total volume of make up solution of sample (sample digestion solution), mL
D.F. = Dilution Factor of digestion solution
D.W. = Dry Weight of Sample, g
Let us consider the readings below
Average C1 = (0.2278 + 0.2260 + 0.2279 + 0.2295) / (4) = 0.2278
Average C2 = (0.0003 + 0.0003 + 0.0003 + 0.0002) / (4) = 0.000275
V = 50 mL (given above)
D.W = 0.2g
D.F = 1
Substituting these values in the above equation,
Concentration of mercury, mg/kg = {(0.2278 – 0.000275) (50) (1)} / (0.2)
= 56.88125 mg/kg.
Characteristic concentration is given as 0.25 mg/L
Uncertainty in Hg levels = {(0.25 - 0.235) / (0.25)} 100
= 6%
SnCl2 acts as a reducing agent in this experiment. It reduces the ironic mercury to metallic mercury (Addition of SnCl2 reduces the mercury in the sample to elemental mercury).
K2Cr2O7 is added to remove sulphide, which is main interference substances.
Bibliography
A. Bartha, and K. Ikrenyi, “Interfering Effects on the Determination of Low Concentrations of Mercury in Geological Materials by Cold-Vapor Atomic Absorption Spectrometry”, Analytical Chimica Acta, 139, 329-332 (1982).
R. B. Band, and N. M. Wilkinson, “Interferences in the Determination of Mercury in Mineralized Samples by the Wet Reduction-Flameless Atomic Absorption Method”, Journal of Geochemical Exploration, 1, 195- 198 (1972).
S. R. Koirtyohann, and Moheb Khalil, “Variables in the Determination of Mercury by Cold Vapor Atomic Absorption”, Analytical Chemistry, 48, 136-139 (1976).
W. M. Johnson, and J. A. Maxwell, Rock and Minerals Analysis, 2nd Edition: John Wiley and Sons, New York, N.Y., p 489 (1981).
W. R. Hatch, and W. L. Ott, “Determination of Submicrogram Quantities of Mercury by Atomic Absorption Spectrophotometry”, Analytical Chemistry 40, 2085-2087 (1968).
